clientupdate: use SPKsVersion instead of Version (#9118)

Top-level Version in pkgs response is not always in sync with SPK
versions, especially on unstable track. It's very confusing when the
confirmation prompt asks you "update to 1.49.x?" and you end up updating
to 1.49.y.
Instead, grab the SPK-specific version field.

Updates #cleanup.

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/clientupdate.go

@@ -204,15 +204,12 @@ func (up *updater) updateSynology() error {
 	if err != nil {
 		return err
 	}
-	if latest.Version == "" {
-		return fmt.Errorf("no latest version found for %q track", up.track)
-	}
 	spkName := latest.SPKs[osName][arch]
 	if spkName == "" {
 		return fmt.Errorf("cannot find Synology package for os=%s arch=%s, please report a bug with your device model", osName, arch)
 	}
 
-	if !up.confirm(latest.Version) {
+	if !up.confirm(latest.SPKsVersion) {
 		return nil
 	}
 	if err := requireRoot(); err != nil {
@@ -832,10 +829,15 @@ func LatestTailscaleVersion(track string) (string, error) {
 type trackPackages struct {
 	Version         string
 	Tarballs        map[string]string
+	TarballsVersion string
 	Exes            []string
+	ExesVersion     string
 	MSIs            map[string]string
+	MSIsVersion     string
 	MacZips         map[string]string
+	MacZipsVersion  string
 	SPKs            map[string]map[string]string
+	SPKsVersion     string
 }
 
 func latestPackages(track string) (*trackPackages, error) {